William Marshall was born in 1144 in Pembroke, Pembrokeshire, Wales and died on May 14, 1219 in Caversham Manor, England at the age of 75. He married Isabel Fitzgilbert De Clare, daughter of Richard Fitzgilbert De Clare and Aoife Macmurrough, in August 1189 in London, England. She was born in 1174 in Pembroke, Pembrokeshire, Wales and died in 1220 in Pembroke, Pembrokeshire, Wales at the age of 46.

Isabel Marshall, countess, daughter of William Marshall and Isabel Fitzgilbert De Clare, was born on October 9, 1200 in Pembroke Castle, Pembrokeshire, Wales and died on January 15, 1239 in Berkhamstead, Hertfordshire, England at the age of 38. She married Gilbert De Clare, son of Richard De Clare and Amicia Fitzrobert, on October 9, 1217 in England. He was born in 1182 in Hertford, Herfordshire, England and died on October 25, 1230 in Penrose, Brittany, France at the age of 48.

Isabel De Clare, daughter of Gilbert De Clare and Isabel Marshall, was born on November 2, 1226 and died in 1254 at the age of 27. She married Robert Bruce, son of Robert Bruce and Isabella Of Huntingdon, in May 1240. He was born in 1210 in Annandale, Dumfriesshire, Scot and died on March 31, 1295 in Priory, Lochmaben, Dumfriesshire, Scot at the age of 85.

Richard De Clare, Earl Of Gloucester, son of Gilbert De Clare and Isabel Marshall, was born on August 4, 1222 and died on July 15, 1262 at the age of 39. He married Maud De Lacy, Countess Of Gloucester on January 25, 1237 in Lincoln, Lincolnshire. She was born in 1223 in Abt. Lincoln, Lincolnshire, England.